Bath for and process of electrodeposition of metal



as r uniform coating is obtained, and which has a Patented Dec. 8, 1936 UNITED STATES na'rn ron m moons s or mononnrosrnou or man Julep]! Schnlein, will, Ill.

No Drawing. Application September 10, 1931,

serial No. 562,201

3 Claims. ((71. 204-1) The present invention relates to improvement in the electro-deposition of metal, and has specific reference to a new and improved bath for of the important objects of the present .invention resides in the utilization of the electro-deposition of metal on off-sized metal parts, and particularly precision tools and: machine parts, such as hobs, thread mills, milling cutters, cams, etc. Thus, where such parts, as for example the bore of a hob, are oversize, they need not be scrapped, but can be brought baclc to size by depositing a plate of the desired thickness. Chromium plate is particularly adapted for this purpose. Metal lost by wear can be replaced in the same manner. Also, hard wearing surfaces can be provided on relatively soft parts. Preferably, a thicker layer of metal than necessary is electro-depcsited, and the excess metal is removed as by grinding or lapping.

Heretofore, it has been found that relatively heavy coats of chromium plate, for example in excess of .002", tend to chip and crack. Another object resides in the provision of a new and improved process of electro-deposition by which chromium plate many times thicker than .002" that will not chip or crack can be deposited, thereby adapting chromium plating successfully to the foregoing uses.

. A further object resides in the provision of a novel process of electro-deposition by which a high rate of deposition so as to enable the expeditious and economical handling of a large number of parts.

More specifically, the invention contemplates the use of an electrolytic bath having a high acid concentration and the property of stabilizing the critical point, and the use of high current densities to increase the throwing power of the bath and the rate of deposition.

In carrying out the process, the bath employed comprises chromic acid, and a suitable sulphate or any chromium plating solution. The chromic acid concentration may be varied over a wide range, and for deposition on steel preferably is about 4.38 M. The ratio between the weight of CIO; and S04 is also subject to considerable variation, and the best results are obtained when the ratio of'CrOz molarity to S04 normality is within the range of 50 to 200, i. e.,

from a ratio of 50 to 1 to a ratio of .200 to l. The precise ratio depends on the type of plate desired, 1. e. bright, dull, deep throwing, etc. For deposition on steel, a ratio of 125 preferably is used. Preferably, the sulphate is added, at least in part, as sulphuric acid (H1804) because of its known purity, but may also be added in the form of chromium sulphate. The high acid concentration and high current density is particularly advantageous for plating off-size metal parts since the rate of deposition is high and since the appearance is not of any great consequence.

In the past, the critical point has been sub- I ject to considerable fluctuation, thus often preventing the current density from being uniform over the entire eflective surface of the cathode. As a result, it has been difllcult to obtain a coating of uniform depth, color and hardness. I have found that voltage fluctuations of the critical point are substantially eliminated upon adding a small amount of cobalt in the form of a suitable soluble salt to the bath. Preferably, the cobalt is added in the form of cobalt sulphate. The cobalt content may be anything up to concentrations producing alloys. In the present instance, the cobalt sulphate is added in such amounts asto furnish one-half of the sulphate (S04) content of the bath. The use of a. small amount of cobalt. as cobalt sulphate or its equivalent in solution, tends to stabilize the critical point and to increase the useful plating range. The cobalt appears to have a catalytic effect on the hydrogen film present on the cathode.

The high acid concentration permits a high current density, and hence imparts deep throwing properties and a high rate of deposition. The current density is an important factor in determining the physical properties of the deposited metal, for example imparting extreme hardness at 4 amperes per square inch. believed that the high current density also tends to cause the deposit of hydrogen on the cathode in an amount sufiicient to reduce the adjacent layer or film of electrolyte and thus allow the deposition to proceed.

With the part to be coated mounted in the electrolytic bath as the cathode, a suitable direct current is impressed across the electrodes. As a result, plating will occur. It will be understood that in the course of the process direct current alone may be used as is common in the art, or, if desired, both direct current and alternating current may be used as hereinafter set Itis.

i'orth. Thus, alternating current having a low voltage and high amperage may be superimposed on the direct current for a predetermined period. The ratio of alternating current to direct current is subject to variation. Thus, the ratio may be such that the lowest resultant current density is below the critical point of cathode polarization. In this instance, the voltage drops through the critical range into the passive zone. The alternating current produces a slight etching effeet on the surface of the metal to be plated, and keeps the surface from being oxidized. No etching occurs above the critical point.

The current ratio may also be such that the lowest resultant current density may be negative with respect to the cathode polarization so that the cathode will become an anode during part of the alternating cin'rent cycle. Finally, the alternating current may be such that the lowest resultant current density is always above the critical point of cathode polarization. It will be evident that any combination of the foregoing may be utilized in series.

After the surface of the cathode has been etched, only the direct current is impressed across the electrodes. Since the alternating current kept the cathode clean, there is no layer of oxide on the surface to be coated when changing to the direct current, and hence immediate deposition occurs.

The rapid deposition gives a close intimate contact which is considerably enhanced by the uneven etching due to the non-homogeneous structure of the steel. Thus, a very adherent plate is obtained.

Also, the alternating current may be superimposed on the direct current during the entire process, but in this event is varied so that the resultant during the initial period dips below the critical point and during the final period never passes below the critical point. Or, alternating current alone may be utilized during the initial period, and then direct current alone may be used during the final period.

It will be understood that it is frequently desirable to prevent certain areas from being plated. Thus, in plating the oversize bore of a hob, no plating is desired on the ends and the outer periphery of the hob. The exterior surfaces may thus be covered with a suitable substance that will keep them free from plating and at the same time from being stained or attacked by the solution.

I claim as my invention:

1. The process of electro-depos iting chromium which consists in passing a current of approximately 4 amperes per square inch through an aqueous chromium plating bath containing approximately 4.38 M chromium trioxide, chromium sulphate and cobalt sulphate, the ratio of the chromium trioxide molarity to sulphate normality being within the range of from 50 to 200, the cobalt sulphate being present in an amount to supply one-half of the sulphate content of the bath and insufllcient to produce a cobalt alloy in an appreciable amount, and maintaining the composition of the bath sub stantially unchanged during the electrolysis.

2. A bath for the electro-deposition of chrmium containing approximately 4.38 M chromic acid and a sulphate radical bearing a molarity ratio to the chromic acid of from .02 to .005, ap-

proximately one-half of the sulphate radical being present in the form of cobalt sulphate.

3. A bath for the electro-deposition of chr0- mium containing approximately 4.38 M chromic acid and a sulphate radical bearing a molarity ratio to the chromic acid of from .02 to .005, part of said sulphate radical being present in the form of cobalt sulpha JOSEPH SCHULEIN. 

